To detect an occurrence of a side impact traffic collision, it is proposed to install a pressure sensor in a hollow body defined in the inside of a vehicle door and measure a change of an air pressure. This kind of pressure sensor is for example disclosed in Japanese Patent No. 2654428 (U.S. Pat. No. 5,748,075).
In such a pressure sensor, a detecting part for detecting the air pressure is arranged in a resinous housing. The housing is formed with a passage through which air in the hollow body is introduced toward the detecting part. Incidentally, water or oil is likely to adhere to an inner wall of the passage due to condensation. If such a water or oil forms a film in the passage due to a surface tension and blocks the passage, detecting sensitivity of the sensor is likely to deteriorate.